Field of the Disclosure
This invention is directed to a turbocharging system for an internal combustion engine and more particularly to a variable turbine geometry (VTG) vane pack assembly having a single-axle, self-centering pivot feature that provides braking capability and durability to the VTG vane pack assembly while allowing for thermal distortion.
Description of Related Art
A turbocharger is a type of forced induction system used with internal combustion engines. Turbochargers deliver compressed air to an engine intake, allowing more fuel to be combusted, thus boosting the horsepower of the engine without significantly increasing engine weight. Thus, turbochargers permit the use of smaller engines that develop the same amount of horsepower as larger, normally aspirated engines. Using a smaller engine in a vehicle has the desired effect of decreasing the mass of the vehicle, increasing performance, and enhancing fuel economy. Moreover, the use of turbochargers permits more complete combustion of the fuel delivered to the engine, which contributes to the highly desirable goal of a cleaner environment.
Turbochargers typically include a turbine housing connected to the exhaust manifold of the engine, a compressor housing connected to the intake manifold of the engine, and a center bearing housing disposed between and coupling the turbine and compressor housings together. The turbine housing defines a generally annular chamber, consisting of a scroll or volute, surrounds the turbine wheel and receives exhaust gas from an exhaust supply flow channel leading from the exhaust manifold of the engine. The turbine assembly generally includes a throat that leads from the chamber into the turbine wheel. The turbine wheel in the turbine housing is rotatably driven by an inflow of exhaust gas supplied from the exhaust manifold. A shaft, rotatably supported in the center bearing housing, connects the turbine wheel to a compressor impeller in the compressor housing so that rotation of the turbine wheel causes rotation of the compressor impeller. The shaft connecting the turbine wheel and the compressor impeller defines a line which is the axis of rotation. Exhaust gas flows the annular turbine chamber, consisting of a scroll or volute, through the throat, to the turbine wheel, where the turbine wheel is driven to spin at extremely high speeds by the exhaust gas. A turbine flow and pressure control means is used to adjust exhaust gas backpressure and turbocharger speed. As the turbine wheel spins at extremely high speeds, the turbine extracts power from the exhaust gas to drive the compressor. The compressor draws in ambient air through an inlet of the compressor housing and the ambient air is compressed by the compressor wheel and is then discharged from the compressor housing to the engine air intake. Rotation of the compressor impeller increases the air mass flow rate, airflow density and air pressure delivered to the cylinders of the engine via the engine intake manifold thus boosting an output of the engine, providing high engine performance, reducing fuel consumption, and environmental pollutants by reducing carbon dioxide (CO2) emissions.
Turbochargers by design operate optimally a limited range of operating conditions. A large turbine may operate optimally at higher air mass flow rates. However, at low air mass flow rates, a large turbine is not efficient, and is unable to quickly spin up to meet the demand for boost, a phenomenon referred to as turbo lag. On the other hand, a small turbine may provide good boost at lower air mass flow rates. But a small turbine can choke when subjected to higher air mass flow rates. For this reason, small turbines may be equipped with bypass as a simple form of boost pressure control. For a turbine equipped with bypass, the turbine size is chosen such that torque characteristic requirements at low engine speeds can be met and good vehicle driveability achieved. With this design, more exhaust gas than required to produce the necessary boost pressure is supplied to the turbine shortly before the maximum torque is reached. Once a specific boost pressure is achieved, part of the excess exhaust gas flow is fed around the turbine via a bypass. The wastegate which opens or closes the bypass is usually operated by a spring-loaded diaphragm in response to the boost pressure. However, bypassing the turbine means that some of the exhaust energy is wasted and not recovered.
Variable turbine geometry allows the turbine flow cross-section to be varied in accordance with the engine operating point. This allows the entire exhaust gas energy to be utilised and the turbine flow cross-section to be set optimally for each operating point. As a result, the efficiency of the turbocharger and hence that of the engine is higher than that achieved with the bypass control. See Mayer “Turbochargers, Effective Use of Exhaust Gas Energy”, Verlag Moderne Inudstrie, 2nd Revised Edition 2001. Variable guide vanes between the volute housing and the turbine wheel have an effect on the pressure build-up behavior and, therefore, on the turbine power output. At low engine speeds, the flow cross-section is reduced by closing the guide vanes. The boost pressure and hence the engine torque rise as a result of the higher pressure drop between turbine inlet and outlet. At high engine speeds, the guide vanes gradually open. The required boost pressure is achieved at a low turbine pressure ratio and the engine's fuel consumption reduced. During vehicle acceleration from low speeds the guide vanes close to gain maximum energy of the exhaust gas. With increasing speed, the vanes open and adapt to the corresponding operating point.
Today, the exhaust gas temperature of modern high-output diesel engines amounts to up to 830° C. The precise and reliable guide vane movement in the hot exhaust gas flow puts high demands on materials and requires tolerances within the turbine to be exactly defined. Irrespective of the turbocharger frame size, the guide vanes need a minimum clearance to ensure reliable operation over the whole vehicle lifetime.
Typically, the adjustable guide vanes of a VTG are pivotably mounted within the turbine housing between a pair of vane rings (upper and lower) and/or a nozzle wall. The adjustable guide vanes are pivoted to control the exhaust gas backpressure and the turbocharger speed by modifying the velocity or direction of exhaust gas flow to the turbine wheel. At lower exhaust gas air mass flow rates the adjustable guide vanes may be moved to a relatively closed position, creating a smaller passage for the flow of exhaust gas. Thereby, the VTG simulates a small turbine, able to achieve higher rotational speeds, even with lower exhaust gas availability. On the other hand, when the engine is at higher speed, exhaust gas air mass flow rate is high. Therefore, the adjustable guide vanes may be opened, creating a larger passage for the flow of exhaust gas and an appropriate amount of boost as needed. The ability of the adjustable guide vanes to open and close allows the turbocharger to operate under a wider range of conditions to meet engine demands. By comparison with bypass control, the VTG utilizes the entire exhaust gas energy, so that the efficiency of the exhaust gas turbocharger, and thus the engine, is enhanced.
VTG turbochargers generally employ at least three fasteners such as studs, bolts, or studs with nuts, to secure the pair of vane rings (i.e. an upper vane ring and a lower vane ring) to the turbine housing such that the turbine housing assembly surrounds the pair of vane rings. The fasteners pass through both of the vane rings to clamp the upper vane ring to the lower vane ring and the lower vane ring to the turbine housing. Any exhaust gas bypassing the vane and flowing through the gap between vane and vane rings reduces efficiency of the VTG. Thus, in order for the vanes to optimally control flow of exhaust gas, the gap between vanes and vane rings must be very small. For the vanes to pivot with such small clearance, the VTG vane assembly must be mounted to the turbine housing with a high degree of geometric parallelism. This parallelism must be maintained as the turbine is subject to a very broad temperature range. Different components are made of different metals, which have different thermal coefficients of expansion. The turbine housing undergoes a certain amount of deformation across temperature ranges due to differential thermal expansion. Deformation of the turbine housing causes the securing mechanisms/fasteners to loose geometric parallelism, so that the vanes and moving components can no longer freely pivot and thus stick or lock-up. Loss of parallelism of the securing mechanisms/fasteners also generates high stress in the securing mechanisms/fasteners, which may lead to failure or breakage of the securing mechanisms/fasteners. Distortion of the vane rings may leads to unusual wear patterns or generates unwanted clearances, which further reduce the aerodynamic efficiency of the turbocharger.
Thus, there is a need for a VTG assembly that allows the vane ring assembly to be positioned and function within the turbine housing. There is a further need to account for thermal growth and distortion of the turbine housing and/or vane ring assembly while maintaining the position of the components of the vane assembly with respect to one another, optimizing peak efficiency. There is a yet an additional need for such a system and method that is cost effective, dependable, and that facilitates an ease of manufacture, assembly and/or disassembly.